The present invention relates to a color cathode ray tube, and more particularly to an electron gun for a color cathode ray tube having double dynamic quadruple pre-focus lenses.
Generally, an electron gun for a cathode ray tube is installed in the neck portion of a funnel to face a screen formed on a panel. In order to improve the resolution of the cathode ray tube, an electron beam emitted from the electron gun is precisely guided to the target position, and particularly, the size of the electron beam spot formed on the screen should be as small as possible. However, since the screen of the cathode ray tube consists of locally varying geometrical curvatures and, at the same time, the R, G and B electron beams proceed in one plane and are initially separated from one another by a predetermined distance, a beam spot of the three electron beams has differing sizes and shapes when respectively formed in the center and peripheral portions of the screen.
Particularly, since the distance from the electron gun to the peripheral portion of the screen is further than that from the electron gun to the center portion of the screen, the focal distance of three electron beams will be different. This difference in focal distance creates a distorted beam spot when the electron beam is guided to the peripheral portion of the screen by the magnetic field of a deflection yoke. The electron beam spot of the peripheral portion of the distorted screen is horizontally elongated. More particularly, a halo is generated around the beam spot, so that a sharp picture cannot be formed. Therefore, the cathode ray tube having such a beam landing structure cannot generate a good picture.
FIG. 1 illustrates a conventional electron gun suggested to improve the aforementioned problem. The electron gun is a dynamic focusing type having a quadruple pre-focus lens whose intensity is changed dynamically. A cathode 2, a control electrode 3 and a screen electrode 4 are placed in the front part of an electron gun 1, composing the prepositioned triode as a source for generating electron beams. First, second, third and fourth focus electrodes 5, 6, 7 and 8 are installed in order, forming the pre-focus lenses of a primary lens system which accelerates and focuses electron beams. A final accelerating electrode 9 is positioned adjacent to fourth focus electrode 8, constituting a main lens with fourth focus electrode 8.
In a conventional electron gun, three horizontally elongated electron beam passing holes 7H are arranged in-line on electron beam outlet 7b of third focus electrode 7, and three vertically elongated electron beam passing holes 8G are arranged in-line on electron beam inlet 8a of fourth focus electrode 8 which is opposed to electron beam outlet 7b. A predetermined static screen voltage Vs is supplied to screen electrode 4 and second focus electrode 6, while a static focus voltage Vf, having a higher potential than that of screen voltage Vs, is supplied to first focus electrode 5 and third focus electrode 7. A parabolic dynamic focus voltage Vd, having a minimum potential which is equal to static focus voltage Vf, is supplied to fourth focus electrode 8 where a deflection signal is synchronized. A static anode voltage Ve having a higher potential than that of focus voltage Vf is supplied to second focus voltage 6.
According to the above voltage supplying structure, a static pre-focus lens 40 of a uni-potential type is formed by first, second and third focus electrodes 5, 6 and 7. A dynamic quadruple pre-focus lens 50 of a bipotential type is formed between third and fourth focus electrodes 7 and 8, while a dynamic main focus lens 60 is formed between fourth focus electrode 8 and final accelerating electrode 9.
Accordingly, thermions emitted from cathode 2 pass through control electrode 3 and screen electrode 4 and form an electron beam which passes through the main lens system and is focused and accelerated, and proceeds toward the screen. Here, when the electron beam is projected to the central part of the screen formed inside the panel, dynamic focus voltage Vd, synchronized with the deflection signal, is equipotential with the static focus voltage Vf, so that lens 50 cannot be formed between third and fourth focus electrodes 7 and 9. Therefore, the electron beam passes through final main lens 60 formed by fourth focus electrode 8 and final accelerating electrode 9 without any effect, and is finally accelerated, focused and guided to the central part of the screen. Since the electron beam is not influenced by dynamic lens 50, a good beam spot having a round cross section is formed.
Also, since dynamic focus voltage Vd is synchronized with the deflection signal, fourth focus electrode 8 has a potential which is greater than static focus voltage Vf when the electron beam is deflected towards peripheral parts of the screen and dynamic quadruple pre-focus lens 50 is formed between third and fourth focus electrode 7 and 8. Accordingly, the electron beam is projected through main focus lens 60 in such a manner that its cross section is vertically elongated. At this time, the intensity of the main focus lens is changed by dynamic focus voltage Vd, so that the intensity of the main focus lens is weaker when the electron beam is guided toward the peripheral part of the screen than when it is guided toward the central part. Therefore, when passing through the main focus lens, the vertically elongated electron beam has a relatively weak focus and acceleration, so that the focal distance becomes longer than when it is guided toward the central part of the screen. As a result, if the vertically elongated electron beam lands on the periphery of the screen, a beam spot having a relative small halo is formed, as in the case when the electron beam lands on the central part of the screen.
However, since in such a conventional electron gun as mentioned above, the intensity of the final main focus lens is changed by the dynamic focus electrode the electron beam passing through the intensity-altered main lens has a different focus acceleration according to its landing position. Therefore, when the electron beam is deflected to the peripheral part of the screen and vertically elongated by the dynamic quadruple pre-focus lens the electron beam undergoes weak focusing and acceleration. Accordingly, after the electron beam passes through the deflection magnetic field of the deflection yoke, due to the influence of the non-uniform deflection magnetic field exerting horizontal focusing and diverging forces, a horizontally elongated beam spot is formed having a different size than the electron beam spot formed on the center of the screen. As a result, beam spots having relatively small halos can be formed on the screen as a whole, but the beam spot sizes are, as a rule, not uniform on the screen, and a good quality picture is difficult to realize.